Fiber reinforced composite materials consisting of reinforcing fibers and a matrix resin are widely used for airplanes, automobiles and other industrial applications, since they are excellent in mechanical properties. In recent years, as the fiber reinforced composite materials may become more widely adopted, they are required to have more excellent properties to meet more severe requirements. To let the fiber reinforced composite materials manifest their mechanical properties and durability sufficiently, it is important to decrease the defects resulting in the decline of strength. Especially for structural materials of airplanes, in view of weight reduction, it is more frequently practiced to use a fiber reinforced composite material as skin panels to be incorporated into a honeycomb sandwich panel. Honeycomb cores are made of aramid or aluminum. In particular, it is general practice to produce a honeycomb sandwich panel by laminating prepreg layers on both sides of a honeycomb core, and co-curing to cure the prepreg layers and to bond the prepreg layers and the honeycomb core simultaneously.
In this case, the bonding strength between the honeycomb core and the prepreg layers laminated as skin panels is important. Conventionally it is popular practice to use a structural adhesive film between the honeycomb core and each of the prepreg layers, for co-curing. However, to further reduce the weight of the honeycomb sandwich panel and to reduce the cost of fabricating, it is desired to self-bond the honeycomb core and the prepreg layers without using any adhesive film.
However, if they are bonded without using any adhesive film, the resin existing in the prepreg layers must migrate into the honeycomb core to sufficiently wet the honeycomb walls, instead of the resin in the adhesive films, and it has been a very difficult problem to achieve a high bonding strength. The cured portions of the resin sinking or rising along the honeycomb walls in the thickness direction of the honeycomb core from the laminated prepreg layers are called fillets, and it is difficult to form the fillets sufficiently between the honeycomb core and the skin panels. If the viscosity of the resin is too low, the resin of the top skin panel tends to flow down too much along the honeycomb walls, and as a result, the bonding strength between the top skin panel and the honeycomb core becomes insufficient. On the other hand, if the resin viscosity is too high, the resin cannot sufficiently wet the honeycomb walls, and especially the bonding strength between the bottom skin panel and the honeycomb core is liable to be insufficient.
On the other hand, since the resin existing in the prepreg layers must be distributed toward the honeycomb core walls, the absolute amount of the resin in the laminated prepreg layers becomes insufficient, and, disadvantageously, pores are likely to be formed in the skin panels. In the case of honeycomb structure, since the pressure for fabricating it does not act on the prepreg layers at the portions above and below the hexagonal voids of the honeycomb core, pores are more likely to be formed as compared with a case of fabricating an ordinary prepreg laminate.
Furthermore, conventionally it is popular practice to stick a structural adhesive film on the surface of each prepreg layer to be laminated, and to cure it together with the prepreg layer for decreasing such defects as pits, dents and resin blurs on the surface of the skin panel. However, to further reduce the weight of the honeycomb sandwich panel and to reduce the material cost and fabricating cost, it is desirable to mold a skin panel with a smooth and defectless surface without sticking the adhesive film to the surface. However, if no adhesive film is used, the amount of the resin remaining at the surface of the skin panel is relatively less since there is no resin corresponding to the resin of the adhesive film, and it has been a difficult problem to achieve a high grade surface condition.
Prior art concerning prepregs and matrix resins with carbon fibers as reinforcing fibers intended to be used for honeycomb fabricating include the following.
U.S. Pat. No. 4,500,660 discloses an epoxy resin composition prepared by adding dicyandiamide to a specific epoxy resin, a reaction product of butadiene acrylonitrile copolymer with functional groups at both the ends and an epoxy resin. The object of the invention is to improve the self adhesiveness between the prepreg layers and the honeycomb and the interlayer shear strength of the skin panels.
JP-A-58-82755 states that if a composition consisting of an epoxy resin and a reaction product of a liquid butadiene acrylonitrile copolymer and an epoxy resin, with dicyandiamide and diaminodiphenylsulfone used together as curing agents is used, the self adhesiveness between the prepreg layers and the honeycomb core is excellent, and that especially the bonding strength at high temperature is high, while the honeycomb sandwich panel does not have any defects on the surfaces.
U.S. Pat. No. 5,557,831 states that the use of a highly thixotropic resin as a woven fabric prepreg to be co-cured into a honeycomb effectively lowers the porosity in the skin panel.
On the other hand, what often come into question in the use of a prepreg are the tackiness and drapability of the prepreg. These properties greatly affect the working convenience in handling the prepreg.
If the tackiness of a prepreg is too small, the overlapped and pressed prepreg is soon peeled, to render inconvenient the laminating operation. In this case, the working environment temperature must be raised to obtain moderate tackiness. On the contrary, if the tackiness of the prepreg is too large, the prepreg overlapped in error adheres due to its own weight, and later peeling for correction becomes difficult.
If the drapability of the prepreg is poor, the lamination work efficiency remarkably declines since the prepreg is hard, and in addition, the laminated prepreg does not accurately suit the curved surface of a mold or the form of a mandrel, and is thus creased or its reinforcing fibers broken, generating defects in the fabricated product. Also in this case, the working environment temperature must be kept high, and since it is difficult to keep balance with the tackiness, this is a very great problem in fabrication.
The tackiness and drapability of a prepreg are mainly dominated by the viscoelasticity of the matrix resin. In general, the viscoelasticity of an epoxy resin greatly depends on the temperature, and if the working environment temperature changes, tackiness and drapability change, preventing application as the case may be.
Furthermore, even if a relatively large amount of a resin exists at the surface to provide moderate tackiness immediately after production of a prepreg, the resin sinks inside with the lapse of time, in a temporal tendency to gradually decrease the tackiness. If the resin viscosity is relatively high, the temporal change of tackiness tends to be small, but on the other hand, drapability is likely to be low. That is, if conventional resins are used, there is a problem that either tackiness or drapability becomes poor.
For improving the tackiness, etc. of a prepreg, it is known to add a high polymer such as a thermoplastic resin or elastomer to an epoxy resin. For example, U.S. Pat. No. 4,859,533 discloses a method of adding polyvinyl formal resin. However, if a high polymer is added in a large amount, the resin viscosity rises, to lower drapability. Because of this restriction, especially for a prepreg using carbon fibers with a high elastic modulus, it has been difficult to find a resin satisfactory both in tackiness and drapability.
U.S. Pat. No. 5,030,698 discloses an epoxy resin composition which is composed of a segmented copolymer consisting of a liquid copolymer and a copolyester, copolyamide, etc. based on an epoxy resin, butadiene and acrylonitrile, to be used as a structural hot melt adhesive, matrix resin or surface paint. As in the above case, the problem of improving tackiness without sacrificing drapability and impregnability is not discussed at all, and no means for solving it is suggested. In addition, if such a liquid copolymer based on butadiene and acrylonitrile is added, the heat resistance and elastic modulus of the cured resin composition decline, and the fiber reinforced composite material obtained by curing a prepreg with such a resin composition as the matrix resin does not have properties such as heat resistance and 0.degree. compressive strength which are sufficiently good.
International Application WO 96/02592 states that a prepreg in which an epoxy resin composition containing a polyamide based or polyester based thermoplastic elastomer is used as the matrix resin is excellent both in drapability and tackiness. However, the improvement of the surface smoothness of a honeycomb sandwich panel is not discussed at all, and no means for solving the problem of providing it is suggested.